Sheet carrying device

ABSTRACT

A sheet carrying device includes: a sheet carrying unit having a first sheet carrying unit and a second sheet carrying unit, at least one of which has a sheet carrying force changing along a direction orthogonal to a traveling direction of a sheet, the first sheet carrying unit carrying the sheet in the traveling direction, the second sheet carrying unit carrying the sheet in the traveling direction together with the first sheet carrying unit; and a control unit which controls the carrying by the first sheet carrying unit and the carrying by the second sheet carrying unit on the basis of a result of detection from a detection unit which detects an inclination of the sheet that is traveling.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Provisional U.S. Application 61/142,071 filed on Dec. 31, 2008, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a sheet carrying device which carries a sheet and an image forming apparatus having the sheet carrying device, and particularly to a carrying operation to correct skewing of a sheet.

BACKGROUND

An image forming apparatus has a sheet carrying device which corrects skewing of a sheet to be supplied to a printer unit, scanner unit or the like. For example, in a sheet carrying device, skewing of a sheet is detected by a sensor and a difference in rotation speed is given between two left and right sets of carrying rollers in accordance with the result of detection, thus correcting the quantity of skew of the sheet. Also, for example, JP-A-2000-335787 or JP-A-2000-34042 discloses a device which adjusts the space between left and right carrying rollers in order to cope with various sheet widths. However, the conventional device requires a bulky dedicated adjustment mechanism in order to adjust the space between the left and right carrying rollers.

Development of a sheet carrying device which copes with various sheet widths and securely corrects skewing of a sheet without using any bulky adjustment mechanism is demanded.

SUMMARY

According to an aspect of the invention, skewing of sheets of various width sizes is accurately corrected by a small-size, light-weight device without using any bulky structure.

According to one embodiment of the invention, a sheet carrying device includes: a sheet carrying unit having a first sheet carrying unit and a second sheet carrying unit, at least one of which has a sheet carrying force changing along a direction orthogonal to a traveling direction of a sheet, the first sheet carrying unit carrying the sheet in the traveling direction, the second sheet carrying unit carrying the sheet in the traveling direction together with the first sheet carrying unit; and a control unit which controls the carrying by the first sheet carrying unit and the carrying by the second sheet carrying unit on the basis of a result of detection from a detection unit which detects an inclination of the sheet that is traveling.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of configuration showing a copier having a registration device according to an embodiment;

FIG. 2 is a schematic view of configuration showing the registration device according to the embodiment, as viewed from top;

FIG. 3 is a schematic explanatory view showing the registration device according to the embodiment, as viewed from a sheet carrying direction;

FIG. 4 is a schematic explanatory view showing a roller that is free with respect to a shaft according to the embodiment;

FIG. 5 is a schematic explanatory view showing the frictional force generated to a sheet when double-driving is performed and the sheet is pulled in the traveling direction, for the explanation of the embodiment;

FIG. 6 is a schematic explanatory view showing the frictional force generated to a sheet when double-driving is performed and the sheet is pulled in the reverse direction of the traveling direction, for the explanation of the embodiment;

FIG. 7 is a schematic explanatory view showing the frictional force generated to a sheet when single-driving is performed and the sheet is pulled in the traveling direction, for the explanation of the embodiment;

FIG. 8 is a schematic explanatory view showing the frictional force generated to a sheet when single-driving is performed and the sheet is pulled in the reverse direction of the traveling direction, for the explanation of the embodiment;

FIG. 9 is a schematic explanatory view showing the shearing force generated to a sheet in the case of double-driven roller pairs only, for the explanation of the embodiment;

FIG. 10 is a schematic explanatory view showing the flexure generated in a sheet by the shearing force, for the explanation of the embodiment;

FIG. 11 is a schematic explanatory view showing the shearing force generated to a sheet in the case where a double-driven roller pair and a single-driven roller pair is combined, for the explanation of the embodiment;

FIG. 12 is a schematic explanatory view showing the detection error of a sensor due to flexure of a sheet, for the explanation of the embodiment;

FIG. 13 is a schematic functional block diagram showing a controller according to the embodiment;

FIG. 14 is a flowchart showing skew correction by the controller according to the embodiment;

FIG. 15 is a schematic explanatory view showing a state where a sheet according to the embodiment passes a first skew detection unit; and

FIG. 16 is a schematic explanatory view showing a state where a sheet according to the embodiment passes a second skew detection unit.

DETAILED DESCRIPTION

Hereinafter, an embodiment will be described. FIG. 1 is a schematic view of configuration showing a copier as an image forming apparatus equipped with a registration device 100 as a sheet carrying device according to the embodiment. In the copier 1, sheets P stacked in a paper supply tray 10 are taken out by a pickup roller 11 and separated one by one by a feed roller 12 and a reverse roller 13. The sheet P is then sent to a guide roller pair 14. The guide roller pair 14 carries the sheet P along a paper supply guide 16.

The copier 1 has the registration device 100 as a sheet carrying device, downstream of the guide roller pair 14. The registration device 100 corrects skewing of the sheet P traveling through the paper supply guide 16 and adjusts the position of the sheet P reaching a transfer device 17.

The copier 1 forms a color image. In the copier 1, for example, four image forming stations 18 which constitute an image forming unit form toner images of four colors of yellow (Y), magenta (M), cyan (C) and black (K) by superimposing these toner images on an intermediate transfer belt 20 as an image carrier. In the copier 1, the transfer device 17 transfers the toner image formed on the intermediate transfer belt 20 to the sheet P.

In the copier 1, a fixing device 22 fixes the toner image to the sheet P by heating and pressurizing. In the copier 1, after the fixing, a paper discharge roller 23 discharges the sheet P to a paper discharge tray 24. In the case of double-side printing, the copier 1 returns the sheet P having the toner image formed on its first side to the paper supply guide 16 via a double-side guide 27 from a gate 26.

As in the formation of the toner image on the first side, in the copier 1, the registration device 100 corrects skewing of the sheet P and adjusts the position of the sheet P in the transfer device 17. In the copier 1, toner images of four colors of yellow (Y), magenta (M), cyan (C) and black (K) for a second side formed by the four image forming stations 18 are superimposed on the intermediate transfer belt 20. In the copier 1, the transfer device 17 transfers the toner image for the second side formed on the intermediate transfer belt 20 to the second side of the sheet P. In the copier 1, the fixing device 22 fixes the toner image to the second side of the sheet P by heating and pressurizing and the paper discharge roller 23 discharges the sheet P having the toner images on both side to the paper discharge tray 24.

FIG. 2 is a schematic view of configuration showing the registration device 100 according to the embodiment, as viewed from above. FIG. 3 is a schematic explanatory view showing the registration device 100 according to the embodiment, as viewed from the carrying direction of the sheet P. The registration device 100 has a front registration roller 110, for example, as a first sheet carrying unit on the front side of the copier 1 with respect to the traveling direction of the sheet P, which is the direction of arrow h. The registration device 100 has a rear registration roller 120, for example, as a second sheet carrying unit on the rear side of the copier 1 with respect to the traveling direction of the sheet P, which is the direction of the arrow h.

In the registration device 100, the front registration roller 110 and the rear registration roller 120 are arranged symmetrically about a center [C] in the traveling direction of the sheet P. The front registration roller 110 and the rear registration roller 120 have a length in the direction of the width of the sheet P. By having a length in the direction of the width of the sheet P, the front registration roller 110 and the rear registration roller 120 carry sheets ranging from a narrow-width sheet to a broad-width sheet. The front registration roller 110 and the rear registration roller 120 carry sheets ranging from, for example, a postcard-size (100 mm×148 mm) sheet to a JIS standard A3-size (297 mm×420 mm) sheet.

The registration device 100 has a front motor 116 which drives the front registration roller 110, and a rear motor 126 which drives the rear registration roller 120. The front registration roller 110 and the rear registration roller 120 are rotationally driven independently of each other.

The registration device 100 has a first skew detection unit 160, which is a detection unit, at a position downstream from the front registration roller 110 and the rear registration roller 120 in the traveling direction of the sheet P but not reaching the transfer device 17. The registration device 100 also has a second skew detection unit 170, which is a detection unit, at a position downstream from the first skew detection unit 160 but not reaching the transfer device 17. The first skew detection unit 160 has four first sensors 160 a, 160 b, 160 c and 160 d arrayed in the direction orthogonal to the traveling direction of the sheet P. The second skew detection unit 170 has four second sensors 170 a, 170 b, 170 c and 170 d arrayed in the direction orthogonal to the traveling direction of the sheet P. The first sensors 160 a, 160 b, 160 c and 160 d and the second sensors 170 a, 170 b, 170 c and 170 d include, for example, optical non-contact sensors.

The registration device 100 has a controller 200 as a control unit which determines the inclination of the sheet P from the results of the detection by the first skew detection unit 160 and the second skew detection unit 170 and controls the front motor 116 and the rear motor 126.

The front registration roller 110 has a first registration roller 111 as a first rotating body and a second registration roller 112 as a second rotating body. The first registration roller 111 and the second registration roller 112 nip and carry the sheet P, as a pair. The first registration roller 111 has, for example, three rollers 113 a, 113 b and 113 c along a first shaft 113. The second registration roller 112 has, for example, three rollers 114 a, 114 b and 114 c along a second shaft 114. The three rollers 113 a, 113 b and 113 c of the first registration roller 111 and the three rollers 114 a, 114 b and 114 c of the second registration roller 112 face each other and form pairs.

The surface of the rollers 113 a, 113 b and 113 c is made of a rubber material such as ethylene propylene rubber (EPDM). The surface of the rollers 114 a, 114 b and 114 c is made of a resin material such as polyacetal (POM) having a smaller coefficient of friction than EPDM. The first registration roller 111 and the second registration roller 112 have different coefficients of friction. In this embodiment, the coefficient of friction of the first registration roller 111 is larger than the coefficient of friction of the second registration roller 112. The material of the surfaces of the first registration roller 111 and the second registration roller 112 is not limited as long as these rollers have different coefficients of friction from each other. For example, the surface of the first registration roller 111 may be made of a rubber material and the surface of the second registration roller 112 may be made of a metal material such as stainless steel (SUS304).

The rollers 113 a, 113 b and 113 c of the first registration roller 111 are fixed to the first shaft 113 and rotate integrally with the first shaft 113. The two rollers 114 a and 114 b near the front side, of the second registration roller 112, are fixed to the second shaft 114 and rotate integrally with the second shaft 114. The one roller 114 c near the center [C] is free with respect to the second shaft 114.

The rear registration roller 120 has a third registration roller 121 as a third rotating body and a fourth registration roller 122 as a fourth rotating body. The third registration roller 121 and the fourth registration roller 122 nip and carry the sheet P, as a pair. The third registration roller 121 has, for example, three rollers 123 a, 123 b and 123 c along a third shaft 123. The fourth registration roller 122 has, for example, three rollers 124 a, 124 b and 124 c along a fourth shaft 124. The three rollers 123 a, 123 b and 123 c of the third registration roller 121 and the three rollers 124 a, 124 b and 124 c of the fourth registration roller 122 face each other and form pairs.

The surface of the rollers 123 a, 123 b and 123 c is made of a rubber material such as EPDM. The surface of the rollers 124 a, 124 b and 124 c is made of a resin material such as POM having a smaller coefficient of friction than EPDM. Thus, the third registration roller 121 and the fourth registration roller 122 have different coefficients of friction. In this embodiment, the coefficient of friction of the third registration roller 121 is larger than the coefficient of friction of the fourth registration roller 122. The material of the surfaces of the third registration roller 121 and the fourth registration roller 122 is not limited. For example, the surface of the third registration roller 121 may be made of a rubber material and the surface of the fourth registration roller 122 may be made of a metal material such as stainless steel (SUS304).

The rollers 123 a, 123 b and 123 c of the third registration roller 121 are fixed to the third shaft 123 and rotate integrally with the third shaft 123. The two rollers 124 a and 124 b near the rear side, of the fourth registration roller 122, are fixed to the fourth shaft 124 and rotate integrally with the fourth shaft 124. The one roller 124 c near the center [C] is free with respect to the fourth shaft 124.

The roller 114 c near the center [C] of the second registration roller 112 and the roller 124 c near the center [C] of the fourth registration roller 122 are attached in a freely movable manner to the second shaft 114 and the fourth shaft 124, respectively, via ball bearings 150, as shown in FIG. 4.

The front motor 116 rotates the first shaft 113 and rotates the second shaft 114 via front gears 117 and 118. That is, the front motor 116 drives the three rollers 113 a, 113 b and 113 c supported by the first shaft 113 and the two rollers 114 a and 114 b supported by the second shaft 114, excluding the roller 114 c near the center [C], which is free with respect to the second shaft 114. Thus, the front motor 116 drives both rollers in a roller pair 130 made up of the roller 113 a and the roller 114 a near the front and in a roller pair 131 made up of the roller 113 b and the roller 114 b. This driving of both rollers is referred to as double-driving. Meanwhile, in a roller pair 132 made up of the roller 113 c and the roller 114 c near the center [C], the front motor 116 drives the one roller 113 c and does not directly drive the roller 114 c. This driving of one roller without driving the other is referred to as single-driving.

As the double-driven roller pairs 130 and 131 and the single-driven roller pair 132 are provided, the coefficient of friction (carrying force) of the front registration roller 110 to the sheet P changes along the direction orthogonal to the traveling direction of the sheet P.

The rear motor 126 rotates the third shaft 123 and rotates the fourth shaft 124 via rear gears 127 and 128. That is, the rear motor 126 drives the three rollers 123 a, 123 b and 123 c supported by the third shaft 123 and the two rollers 124 a and 124 b supported by the fourth shaft 124, excluding the roller 124 c near the center [C] which is free with respect to the fourth shaft 124. Thus, the rear motor 126 double-drives a roller pair 140 made up of the roller 123 a and the roller 124 a near the rear side and a roller pair 141 made up of the roller 123 b and the roller 124 b. Meanwhile, the rear motor 126 single-drives a roller pair 142 made up of the roller 123 c and the roller 124 c near the center [C].

As the double-driven roller pairs 140 and 141 and the single-driven roller pair 142 are provided, the coefficient of friction (carrying force) of the rear registration roller 120 to the sheet P changes along the direction orthogonal to the traveling direction of the sheet P.

In this embodiment, the two roller pairs 130 and 131 on the front side and the two roller pairs 140 and 141 on the rear side are double-driven to securely carry the sheet P, whereas the roller pairs 132 and 142 near the center [C] are single-driven to restrain flexure of the sheet P. As flexure of the sheet P is restrained, reduction in the detection accuracy of the first skew detection unit 160 and the second skew detection unit 170 is restrained.

The principle of restraining flexure by single-driving the roller pairs will now be described. The difference in frictional force generated to the sheet P between the cases of double-driving and single-driving a roller pair 50 including a rubber roller 50 a and a resin roller 50 b having a smaller coefficient of friction than the rubber roller 50 a will be described.

(1) FIG. 5 shows the frictional force in the case where double-driving to drive the rubber roller 50 a in the direction of arrow m and to drive the resin roller 50 b in the direction of arrow n is performed and the sheet P is pulled in the direction of arrow q, which is the traveling direction. A frictional force r1 in the reverse direction of the pulling direction is applied to the sheet P by the rubber roller 50 a and a frictional force r2 that is smaller than the frictional force r1 is applied to the sheet P by the resin roller 50 b.

(2) FIG. 6 shows the frictional force in the case where double-driving to drive the rubber roller 50 a in the direction of arrow m and to drive the resin roller 50 b in the direction of arrow n is performed and the sheet P is pulled in the direction of arrow s, which is opposite to the traveling direction. A frictional force u1 in the pulling direction is applied to the sheet P by the rubber roller 50 a and a frictional force u2 that is smaller than the frictional force u1 is applied to the sheet P by the resin roller 50 b.

(3) FIG. 7 shows the frictional force in the case where single-driving to drive the rubber roller 50 a in the direction of arrow m and to leave the resin roller 50 b free is performed and the sheet P is pulled in the direction of arrow q, which is the traveling direction. The frictional force r1 in the reverse direction of the pulling direction is applied to the sheet P by the rubber roller 50 a. The free resin roller 50 b follows the traveling of the sheet P and rotates in the direction of arrow n. However, the free resin roller 50 b generates little frictional force to the sheet P.

(4) FIG. 8 shows the frictional force in the case where single-driving to drive the rubber roller 50 a in the direction of arrow m and to leave the resin roller 50 b free is performed and the sheet P is pulled in the direction of arrow s, which is opposite to the traveling direction. The frictional force u1 in the pulling direction is applied to the sheet P by the rubber roller 50 a. The free resin roller 50 b follows the traveling of the sheet P and rotates in the direction of arrow n. However, the free resin roller 50 b generates little frictional force to the sheet P.

As described in the above (1) to (4), the frictional force generated to the sheet by the roller pair 50 is smaller at the time of single-driving than at the time of double-driving.

Therefore, when the sheet P is carried with both sides of the sheet P nipped by plural roller pairs, the shearing force generated to the sheet P differs between the case where all the roller pairs are double-driven and the case where a part of the roller pairs is single-driven.

(5) For example, FIG. 9 shows the frictional force generated to the sheet P in the case where all of two front roller pairs 60 a and 60 b and two rear roller pairs 70 a and 70 b are double-driven and the sheet P is carried to the direction of a transfer device 80, with the rotation speed of the rear roller pairs 70 a and 70 b being lower than the rotation speed of the front roller pairs 60 a and 60 b. Frictional forces z1 of substantially the same magnitude are applied to the sheet P from the two front roller pairs 60 a and 60 b having the higher rotation speed. Frictional forces z2 of substantially the same magnitude are applied to the sheet P from the two rear roller pairs 70 a and 70 b having the lower rotation speed. Therefore, the sheet P is pulled by the frictional forces z1 and the frictional forces z2 in the opposite directions to each other and a large shearing force indicated by a chain-dotted line a is generated near the center [C] of the sheet P. This shearing force causes the sheet P to flex near its center [C], as shown in FIG. 10. With the fluctuation in the magnitude of the flexure, the distance between optical non-contact sensors 75 and the sheet P changes and there is a risk that the detection accuracy of the sensors 75 may be lowered.

(6) FIG. 11 shows, with respect to the above (5), the frictional force generated to the sheet P in the case where the one roller pair 60 a close to the front side and the one roller pair 70 a close to the rear side are double-driven while the roller pairs 60 b and 70 b close to the center [C] are single-driven and the sheet P is carried to the direction of the transfer device 80, with the rotation speed of the rear roller pairs 70 a and 70 b being lower than the rotation speed of the front roller pairs 60 a and 60 b. The frictional force z1 and the frictional force z2 are applied to the sheet P from the double-driven roller pair 60 a close to the front side and the double-driven roller pair 70 a close to the rear side, respectively. Meanwhile, a frictional force z3 smaller than the frictional force z1 and a frictional force z4 smaller than the frictional force z2 are applied to the sheet P from the single-driven roller pairs 60 b and 70 b close to the center [C], respectively.

Therefore, the shearing force generated to the sheet P near its center [C], as indicated by a chain-dotted line β, is smaller than the shearing force of the above (5) indicated by the chain-dotted line α. Since the shearing force is smaller, the flexure near the center [C] of the sheet P is smaller.

The flexure generated in the sheet P significantly affects the detection accuracy of the first skew detection unit 160 and the second skew detection unit 170. For example, FIG. 12 shows the influence in the case of using optical non-contact sensors for the first skew detection unit 160 and the second skew detection unit 170. An optical non-contact sensor 76 has a specific detection area. For example, the detection area of the sensor 76 is assumed to be the range indicated by a chain-dotted line γ. When the forward end of the sheet reaches the detection area γ, the sensor 76 receives reflected light from the forward end of the sheet and detects the arrival of the sheet P.

If the sheet is not flexed as indicated by P1, the sensor 76 detects the forward end of the sheet in timing t1. On the other hand, if the sheet is flexed as indicated by P2, the arrival of the forward end of the sheet at the detection area γ is delayed even though the sheet enters the detection position of the sensor 76 in the same skewing state. If the sheet is flexed, the sensor 76 detects the forward end of the sheet in timing t2. If the sheet is flexed, the detection timing is delayed by Δt, compared with the case where the sheet is not flexed. Thus, the detection accuracy of the sensor 76 is lowered. As the flexure of the sheet increases, the delay Δt in detection timing increases. Therefore, in order to enhance the detection accuracy of the sensor 76, the flexure of the sheet needs to be restrained.

Thus, in this embodiment, a combination of double-driven roller pairs and single-driven roller pairs is used as the front registration roller 110 and the rear registration roller 120 and thus the flexure of the sheet is minimized.

The first skew detection unit 160 and the second skew detection unit 170 of the registration device 100 have the first sensors 160 a, 160 b, 160 c and 160 d and the second sensors 170 a, 170 b, 170 c and 170 d arrayed in the direction orthogonal to the traveling direction of the sheet P so that these sensors can detect the passage of sheets ranging from, for example a narrow sheet P of the postcard size to a broad sheet P of the JIS standard A3 size. For example, the space between the first sensors 160 b and 160 c and between the second sensors 170 b and 170 c in the center is a distance W1. The space between the first sensors 160 a and 160 b, 160 c and 160 d, the second sensors 170 a and 170 b, and 170 c and 170 d on both sides is a distance W2. The distance W1 is set, for example, to 75 to 85% of the width of the postcard size. The distance W2 is set, for example, in such a manner that (W1+W2×2) is 75 to 85% of the width where a sheet of the JIS standard A3 size is longitudinally carried.

The controller 200 has a skew determination unit 201 and a drive control unit 202, as shown in FIG. 13. The controller 200 is connected to a CPU 300 which controls the entire copier 1, and realizes a skew determination function and a drive control function. The CPU 300 has a memory 310. The CPU 300 executes programs stored in the memory 310 and thus realizes various functions. The memory 310 can include, for example, a RAM (random access memory), ROM (read only memory), a DRAM (dynamic random access memory), SRAM (static random access memory), VRAM (video RAM) or the like. The memory 310 stores various kinds of information and programs used in the copier 1 including the registration device 100.

The skew determination unit 201 determines the quantity of skew of the sheet P on the basis of the detection of the forward end of the sheet P or the rear end of the sheet P that is acquired from the first skew detection unit 160 and the second skew detection unit 170. The drive control unit 202 separately controls the front motor 116 and the rear motor 126 to reduce the quantity of skew of the sheet P determined by the skew determination unit 201.

Next, the traveling of the sheet P when the position of the sheet P is adjusted by the registration device 100 configured as described above and the sheet P is thus carried to the direction of the transfer device 17, will be described.

At the time of printing, the image forming stations 18 form toner images on the intermediate transfer belt 20. The controller 200 controls the registration device 100 to correct the skewing of the sheet P, adjusts the position of the sheet P, and carries the sheet P to the transfer device 17 synchronously with the arrival of the toner images on the intermediate transfer belt 20 at the transfer device 17. That is, the pickup roller 11 takes out sheets P from the paper supply tray 10 synchronously with the toner images on the intermediate transfer belt 20. The feed roller 12 and the reverse roller 13 separate the sheets P one by one. The guide roller pair 14 sends the sheet P to the registration device 100.

In the registration device 100, the controller 200 controls the driving of the front motor 116 and the rear motor 126, thus controls the rotation of the front registration roller 110 and the rear registration roller 120, and corrects the skewing of the sheet P. If the front registration roller 110 and the rear registration roller 120 have different rotation speeds, the frictional force applied to the sheet P from the front registration roller 110 is different from the frictional force applied to the sheet P from the rear registration roller 120, thus generating a shearing force near the center [C] of the sheet P.

However, in the front registration roller 110, the two roller pairs 130 and 131 close to the front side are double-driven, whereas the roller pair 132 close to the center [C] is single-driven. In the rear registration roller 120, the two roller pairs 140 and 141 close to the rear side are double-driven, whereas the roller pair 142 close to the center [C] is single-driven. Therefore, as in the above description of (6), the shearing force generated to the sheet P is weakened by the single-driven roller pairs 132 and 142 close to the center [C]. Since the shearing force generated to the sheet P is weakened, the flexure of the sheet P is reduced. The error Δt in the detection timing of the sheet P by the first skew detection unit 160 and the second skew detection unit 170 is reduced. Consequently, the first skew detection unit 160 and the second skew detection unit 170 detect the sheet P very accurately.

The first skew detection unit 160 and the second skew detection unit 170 input the result of the detection to the skew determination unit 201 of the controller 200. The control of the front motor 116 and the rear motor 126 by the controller 200 based on the results of the detection by the first skew detection unit 160 and the second skew detection unit 170 is shown in the flowchart of FIG. 14. As the sheet P reaches the first skew detection unit 160, the four first sensors 160 a, 160 b, 160 c and 160 d arrayed in the direction orthogonal to the traveling direction of the sheet P input the results of detection respectively to the skew determination unit 201 in timing of the arrival of the sheet P.

The skew determination unit 201 determines the quantity of skew of the sheet P based on the result of the detection of the forward end of the sheet P from the first skew detection unit 160 (ACT 401).

For example, if the sheet P is skewed as shown in FIG. 15, the first sensor 160 d on the rear side, where the forward end of the sheet comes early, turns on first. After that, the first sensors 160 c, 160 b and 160 a sequentially turn on toward the front side. The skew determination unit 201 determines a quantity of skew (θ1) of the sheet on the basis of the time difference when the first sensors 160 d, 160 c, 160 b and 160 a sequentially turn on from the rear side, the distances W1 and W2 between the first sensors 160 d, 160 c, 160 b and 160 a, and the average carrying speed of the sheet P.

The drive control unit 202 separately controls the front motor 116 and the rear motor 126 and thus controls rotational driving of the front registration roller 110 and the rear registration roller 120 so as to correct the quantity of skew (θ1) determined by the skew determination unit 201 on the basis of the result of the detection by the first skew detection unit 160 (ACT 402).

The drive control unit 202 finds a circumferential speed difference (V1) between the front registration roller 110 and the rear registration roller 120 required for canceling the quantity of skew (θ1) determined by the skew determination unit 201, by the time when, for example, the sheet P reaches the transfer device 17 from the first skew detection unit 160. The drive control unit 202 separately controls the front motor 116 and the rear motor 126 in such a manner that the circumferential speed difference between the front registration roller 110 and the rear registration roller 120 becomes the acquired circumferential speed difference (V1).

As the drive control unit 202 separately controls the front motor 116 and the rear motor 126, the registration device 100 carries the sheet P to the second skew detection unit 170 while correcting the quantity of skew (θ1) of the sheet P.

After that, when the sheet P reaches the second skew detection unit 170, as shown in FIG. 16, the four second sensors 170 a, 170 b, 170 c and 170 d arrayed in the direction orthogonal to the traveling direction of the sheet P input the results of detection respectively to the skew determination unit 201 in timing of the arrival of the sheet P. The skew determination unit 201 determines the quantity of skew of the sheet P based on the result of the detection of the forward end of the sheet P from the second skew detection unit 170 (ACT 403).

As in ACT 401, the skew determination unit 201 determines a quantity of skew (θ2) of the sheet at the second skew detection unit 170 on the basis of the time difference when the second sensors 170 a, 170 b, 170 c and 170 d sequentially turn on according to the inclination of the sheet P, the distances W1 and W2 between the second sensors 170 a, 170 b, 170 c and 170 d, and the average carrying speed of the sheet P.

The drive control unit 202 separately controls the front motor 116 and the rear motor 126 and further controls rotational driving of the front registration roller 110 and the rear registration roller 120 so as to correct the quantity of skew (θ2) determined by the skew determination unit 201 on the basis of the result of the detection by the second skew detection unit 170 (ACT 404).

As in ACT 402, the drive control unit 202 finds a circumferential speed difference (V2) between the front registration roller 110 and the rear registration roller 120 required for canceling the quantity of skew (θ2) determined by the skew determination unit 201. The drive control unit 202 separately controls the front motor 116 and the rear motor 126 in such a manner that the circumferential speed difference between the front registration roller 110 and the rear registration roller 120 becomes the acquired circumferential speed difference (V2). Then, the control by the controller 200 finishes.

The registration device 100 carries the sheet P to the transfer device 17 while correcting the quantity of skew (θ2) of the sheet P by separately controlling the front motor 116 and the rear motor 126. The front motor 116 and the rear motor 126 are controlled on the basis of the result of the detection by the first skew detection unit 160 and further controlled on the basis of the result of the detection by the second skew detection unit 170. The skewing of the sheet P is thus corrected and the position of the sheet P reaching the transfer device 17 is adjusted.

After that, the transfer device 17 transfers the toner images on the intermediate transfer belt 20 to the sheet P that reaches the transfer device 17 synchronously with the toner images on the intermediate transfer belt 20. The copier 1 fixes the toner images onto the sheet P by heating and pressurizing, then discharges the sheet to the paper discharge tray 24 and finishes the printing.

According to the embodiment, in the registration device 100, the roller pairs 132 and 142 close of the center [C], of the front registration roller 110 and the rear registration roller 120 that nip both sides of sheets of various width sizes, are single-driven. Thus, compared with the case where all the roller pairs of the front registration roller 110 and the rear registration roller 120 are double-driven, the shearing force generated to the sheet P by the speed difference of the front registration roller 110 and the rear registration roller 120 is reduced and the flexure of the sheet P is reduced. Consequently, without using a complex mechanism, the detection error in detecting the sheet P by the first skew detection unit 160 or the second skew detection unit 170 can be decreased and the detection accuracy in detecting the sheet P by the first skew detection unit 160 or the second skew detection unit 170 can be improved. On the basis of the highly accurate detection result, the controller 200 can accurately control the front motor 116 and the rear motor 126. The registration device 100 can securely correct the skewing of the sheet P.

The invention is not limited to the above embodiment and various changes and modifications can be made without departing from the scope of the invention. For example, with respect to the sheet carrying unit, it is possible that only the sheet carrying force of one of the first sheet carrying unit and the second sheet carrying unit changes along the direction orthogonal to the traveling direction of the sheet, as long as the flexure of the sheet due to the shearing force generated to the sheet can be prevented. Also, the detection units are not limited to optical non-contact sensors. The number of detection units and the arrangement or the like of detection units are not limited, either. 

1. A sheet carrying device comprising: a sheet carrying unit having a first sheet carrying unit and a second sheet carrying unit, at least one of which has a sheet carrying force changing along a direction orthogonal to a traveling direction of a sheet, the first sheet carrying unit carrying the sheet in the traveling direction, the second sheet carrying unit carrying the sheet in the traveling direction together with the first sheet carrying unit; and a control unit which controls the carrying by the first sheet carrying unit and the carrying by the second sheet carrying unit on the basis of a result of detection from a detection unit which detects an inclination of the sheet that is traveling.
 2. The device according to claim 1, wherein the change in the sheet carrying force is a change in coefficient of friction to the sheet.
 3. The device according to claim 2, wherein as the change in the coefficient of friction of at least one of the first sheet carrying unit and the second sheet carrying unit, the coefficient of friction becomes smaller on a side close to a center in the traveling direction of the sheet.
 4. The device according to claim 2, wherein the first sheet carrying unit has a first rotating body which supports plural rollers along a first shaft, and a second rotating body which has a different coefficient of friction from the first rotating body, supports plural rollers along a second shaft and nips the sheet together with the first rotating body, and the second sheet carrying unit has a third rotating body which supports plural rollers along a third shaft, and a fourth rotating body which has a different coefficient of friction from the third rotating body, supports plural rollers along a fourth shaft and nips the sheet together with the third rotating body.
 5. The device according to claim 4, wherein one having a larger coefficient of friction, of the first rotating body and the second rotating body, has a contact surface to the sheet made of a rubber material, and one having a smaller coefficient of friction has a contact surface to the sheet made of a metal material or resin material, and one having a larger coefficient of friction, of the third rotating body and the fourth rotating body, has a contact surface to the sheet made of a rubber material, and one having a smaller coefficient of friction has a contact surface to the sheet made of a metal material or resin material.
 6. The device according to claim 4, wherein in the rotating body having a larger coefficient of friction, of the first rotating body and the second rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller is free with respect to the shaft, and in the rotating body having a larger coefficient of friction, of the third rotating body and the fourth rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller is free with respect to the shaft.
 7. The device according to claim 6, wherein at least in one of the first sheet carrying unit and the second sheet carrying unit having the changing coefficient of friction to the sheet, the roller that is free with respect to the shaft is located on the side close to the center in the traveling direction of the sheet.
 8. The device according to claim 3, wherein the first sheet carrying unit and the second sheet carrying unit are symmetrical about the center in the traveling direction of the sheet, and both the first sheet carrying unit and the second sheet carrying unit have the coefficient of friction changing along the direction orthogonal to the traveling direction of the sheet, and as the change in the coefficient of friction, the coefficient of friction becomes smaller on the side close to the center of the traveling direction of the sheet.
 9. The device according to claim 8, wherein the first sheet carrying unit has a first rotating body which supports plural rollers along a first shaft, and a second rotating body which has a different coefficient of friction from the first rotating body, supports plural rollers along a second shaft and nips the sheet together with the first rotating body, and the second sheet carrying unit has a third rotating body which supports plural rollers along a third shaft, and a fourth rotating body which has a different coefficient of friction from the third rotating body, supports plural rollers along a fourth shaft and nips the sheet together with the third rotating body.
 10. The device according to claim 8, wherein one having a larger coefficient of friction, of the first rotating body and the second rotating body, has a contact surface to the sheet made of a rubber material, and one having a smaller coefficient of friction has a contact surface to the sheet made of a metal material or resin material, and one having a larger coefficient of friction, of the third rotating body and the fourth rotating body, has a contact surface to the sheet made of a rubber material, and one having a smaller coefficient of friction has a contact surface to the sheet made of a metal material or resin material.
 11. The device according to claim 8, wherein in the rotating body having a larger coefficient of friction, of the first rotating body and the second rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller is free with respect to the shaft, and in the rotating body having a larger coefficient of friction, of the third rotating body and the fourth rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller is free with respect to the shaft.
 12. The device according to claim 11, wherein the roller that is free with respect to the shaft in the first sheet carrying unit and the second sheet carrying unit is located on the side close to the center in the traveling direction of the sheet.
 13. The device according to claim 9, wherein the control unit controls a first driving unit which rotates the first shaft and the second shaft, and a second driving unit which rotates the third shaft and the fourth shaft.
 14. A sheet carrying device comprising: a first sheet carrying unit which nips a sheet between a first rotating body and a second rotating body having a different coefficient of friction from the first rotating body and thus carries the sheet in a traveling direction; a second sheet carrying unit which nips the sheet between a third rotating body and a fourth rotating body having a different coefficient of friction from the third rotating body and thus carries the sheet in the traveling direction together with the first sheet carrying unit; and a control unit which controls the carrying by the first sheet carrying unit and the carrying by the second sheet carrying unit on the basis of a result of detection from a detection unit which detects an inclination of the sheet that is traveling.
 15. The device according to claim 14, wherein the first rotating body has plural rollers along a first shaft orthogonal to the traveling direction of the sheet, and the second rotating body has plural counter-rollers along a second shaft, the plural counter-rollers facing the plural rollers along the first shaft, and the third rotating body has plural rollers along a third shaft orthogonal to the traveling direction of the sheet, and the fourth rotating body has plural counter-rollers along a fourth shaft, the plural counter-rollers facing the plural rollers along the third shaft.
 16. The device according to claim 15, wherein in the rotating body having a larger coefficient of friction, of the first rotating body and the second rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller is free with respect to the shaft, and in the rotating body having a larger coefficient of friction, of the third rotating body and the fourth rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller is free with respect to the shaft.
 17. The device according to claim 16, wherein the roller that is free with respect to the shaft in the first sheet carrying unit and the second sheet carrying unit is located on the side close to the center in the traveling direction of the sheet.
 18. An image forming apparatus comprising: an image forming unit which forms a toner image on an image carrier; a sheet carrying device including a sheet carrying unit having a first sheet carrying unit and a second sheet carrying unit, at least one of which has a coefficient of friction changing along a direction orthogonal to a traveling direction of a sheet, the first sheet carrying unit carrying the sheet in the traveling direction, the second sheet carrying unit carrying the sheet in the traveling direction together with the first sheet carrying unit, and a control unit which controls the carrying by the first sheet carrying unit and the carrying by the second sheet carrying unit on the basis of a result of detection from a detection unit which detects an inclination of the sheet that is traveling; and a transfer unit which transfers the toner image from the image carrier to the sheet carried by the sheet carrying device.
 19. The apparatus according to claim 18, wherein as the change in the coefficient of friction of at least one of the first sheet carrying unit and the second sheet carrying unit, the coefficient of friction becomes smaller on a side close to a center in the traveling direction of the sheet.
 20. The apparatus according to claim 19, wherein the first sheet carrying unit and the second sheet carrying unit are symmetrical about the center in the traveling direction of the sheet, the first sheet carrying unit has a first rotating body which supports plural rollers along a first shaft, and a second rotating body which has a different coefficient of friction from the first rotating body, supports plural rollers along a second shaft and nips the sheet together with the first rotating body, and in the rotating body having a larger coefficient of friction, of the first rotating body and the second rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller located on the side close to the center is free with respect to the shaft, and the second sheet carrying unit has a third rotating body which supports plural rollers along a third shaft, and a fourth rotating body which has a different coefficient of friction from the third rotating body, supports plural rollers along a fourth shaft and nips the sheet together with the third rotating body, and in the rotating body having a larger coefficient of friction, of the third rotating body and the fourth rotating body, all the rollers rotate integrally with the shaft, and in the rotating body having a smaller coefficient of friction, a part of the rollers rotates integrally with the shaft and the remaining roller located on the side close to the center is free with respect to the shaft. 